This application claims the priority of German application 198 52 127.8, filed Nov. 12, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an expansion element for an air conditioner and to a valve unit which can be used particularly for such an expansion element and has a fixed throttle between an upstream valve high-pressure side and a downstream valve low- pressure side.
Air conditioners, as used, for example, in motor vehicles, conventionally contain a refrigerant circulating system which is divided, on one hand, by a compressor and, on the other hand, by an expansion element into a refrigerant high-pressure side and a refrigerant low-pressure side. On the high-pressure side, between the compressor and the expansion element, a heat exchanger is situated for cooling the refrigerant. The heat exchanger operates as a condenser or a gas cooler depending on whether the refrigerant on the high-pressure side is operated in the subcritical or supercritical range.
On the low-pressure side, between the expansion element and the compressor an evaporator is situated over which an air current to be cooled is guided. Conventional expansion elements usually contain a control valve which variably adjusts the passage cross-section of the expansion element as a function of an influencing correcting variable. Typically, the overheating of the refrigerant behind the evaporator is controlled by influencing the refrigerant flow rate. Thus, EP 0 438 625 A2 describes an expansion element with a control valve for which the low-pressure-side refrigerant temperature behind the evaporator is used as a correcting variable.
EP 0 701 096 A2 discloses a vehicle air conditioner which operates preferably with CO2 as the refrigerant. Either a fixed throttle or a control valve is provided as the expansion element. When the control valve is used, a so-called COP control of the CO2 air conditioner is endeavored. The coefficient of performance (COP) , defined as the ratio of the supplied cooling output to the spent power, if possible, is held within the range of a maximum which the COP assumes as a function of the refrigerant pressure on the high-pressure side. For this purpose, the adjustment of the control valve is correlated with the control of a throughput-controllable compressor, whereby the refrigerant flow rate is varied by regulating the refrigerant throughput in the compressor.
EP 0 786 632 A2 discloses a CO2 air conditioner with an expansion element which contains a control valve which is acted upon by the high-pressure-side refrigerant pressure. The differential pressure, which acts upon a movable membrane and which exists between the high-pressure-side refrigerant pressure and the pressure in a closed chamber which is filled with refrigerant and is in a thermal contact with the high-pressure-side refrigerant flowing past, operates as the effective correcting variable. In this manner, the high-pressure-side refrigerant pressure can be varied along a curve which, in the supercritical range, corresponds to a curve of constant refrigerant density, whereby the COP again is to be kept at a maximum.
An object of the present invention is to provide a novel valve unit which, when used in an expansion element for an air conditioner, particularly a CO2 air conditioner, which, on the high-pressure side, also reaches the supercritical range of the used refrigerant, permits a comparatively reliable and energy-optimal control of the refrigerant circulation at relatively low expenditures. The present invention is also based on a novel expansion element by which particularly also a CO2 air conditioner can be controlled in a comparatively easy and reliable manner.
The present invention achieves these objects by providing a valve unit having a fixed throttle between a valve high-pressure side and a valve low-pressure side, and at least one additional component comprising one of a pressure control valve arranged in a bypass line bypassing the fixed throttle and of a control valve influencing the passage cross-section of the fixed throttle, as well as an expansion element having a control valve configured to adjust a flow cross-section of the refrigerant from the high-pressure side to the low-pressure side as a function of an effective correcting variable and to be acted upon by the refrigerant pressure on the low-pressure side or a physical quantity connected therewith as the correcting variable or an expansion element characterized by a valve unit, or configured to be coupled on the valve high-pressure side to a condenser, gas cooler or internal heat exchanger of the air conditioner and on the valve low-pressure side to a low-pressure-side refrigerant flow section of the air conditioner.
The valve unit according to the present invention characteristically contains a fixed throttle between the valve high-pressure side and the valve low-pressure side as well as at least one additional valve component in the form of a pressure control valve which is arranged in a bypass line bypassing the fixed throttle, or in the form of a control valve which influences the passage cross-section of the fixed throttle.
The combined arrangement of the fixed throttle and the pressure control valve is suitable for use as an expansion element, which can be implemented in a comparatively easy manner, specifically also of a CO2 air conditioner which can be operated supercritically on the high-pressure side at least in certain operating situations. The fixed throttle saves the control expenditures which are connected with a control valve and, because of its pressure-dependent throttle characteristic, nevertheless to a certain extent permits a high pressure regulating. The pressure control valve acted upon by the high-pressure-side refrigerant pressure provides a high-pressure safety limitation and is advantageously arranged in an associated bypass line so that it does not at all affect the refrigerant flow through the fixed throttle.
As required, the use of a control valve influencing the passage cross-section of the fixed throttle permits control of the air conditioner which is refined in comparison to the sole use of an uncontrolled fixed throttle. Nevertheless, also in this case, the implementation and control expenditures clearly remain lower than when conventional expansion element control valves are used which have no fixed throttle part and have a partially external control.
A further development of the valve unit has a second fixed throttle connected in parallel to the first. The two fixed throttles preferably have different configurations so that, if an expansion element is used in contrast to a sole fixed throttle, a two-step and therefore refined control of a refrigerant circulation is achievable.
In a further development, the valve unit has, in addition to the two parallel fixed throttles, a pressure control valve in a bypass line bypassing the fixed throttle arrangement as well as a control valve which influences the passage cross-section of one of the fixed throttles. This permits a particularly sensitive control of a refrigerant circulating system still without the implementation and control expenditures of a conventional expansion element control valve complex. The reason is that, because of the presence of the fixed throttles, the control valve used here may have a relatively simple construction and must carry out a comparatively simple control function.
In a still further development of the valve according to the present invention, all valve components are advantageously integrated in a common valve housing so that the valve unit with the several valve components can be implemented as a uniform compact constructional unit.
A valve unit according to the present invention can also have a mechanical control valve which influences the passage cross-section of a fixed throttle and which is acted upon by the pressure of the medium on the low-pressure side as the correcting variable. When the valve unit is used in an expansion element, a simple refrigerant high-pressure control is permitted with the low-pressure-side refrigerant pressure, also called suction pressure, as the correcting variable.
In another valve unit embodiment according to the present invention, a thermal control valve influences the passage cross-section of a fixed throttle and is acted upon by the temperature existing on the low-pressure side as the correcting variable. When the valve unit is used in an expansion element of an air conditioner, a refrigerant high-pressure control is permitted with the low-pressure-side refrigerant temperature, also called suction gas temperature, as the correcting variable.
In a further developed valve unit according to the present invention, a thermal control valve is provided which influences the passage cross-section of a fixed throttle and to whom a heater is assigned by which it can be controlled by an associated heater input quantity as the correcting variable. When the valve unit is used, for example, in an expansion element of a vehicle air conditioner, this correcting variable may be an electric heating current signal which depends on the ambient temperature or the rotational speed of a motor vehicle engine driving the air conditioner compressor. In addition, the thermal control valve can also be influenced by the low-pressure-side refrigerant temperature.
In yet a still further developed valve unit, the low-pressure side of the control valve, which can be influenced by a low-pressure-side physical quantity and acts upon a fixed throttle, is connected by a valve-housing-internal connection line directly with the low-pressure side of the fixed throttle. As the result, influencing of the control valve is implemented by the low-pressure side already in the valve unit itself and must no longer be caused externally so that a single low-pressure connection is sufficient for the valve unit.
The expansion element according to the present invention characteristically contains a mechanical control valve which is acted upon by the low-pressure-side refrigerant pressure as the correcting variable. This permits a refrigerant high-pressure control with the low-pressure-side refrigerant pressure in front of or behind the evaporator which, specifically also for CO2 air conditioners of motor vehicles, on one hand, is found to have relatively low expenditures and, on the other hand, is found to be comparatively reliable and favorable in terms of energy consumption.
The expansion element characteristically contains a valve unit of the above-described types coupled on the valve high-pressure side to the refrigerant high-pressure side of the air conditioner and on the valve low-pressure side to the refrigerant low-pressure side of the air conditioner. As a result of the valve components contained in the valve unit, this expansion element permits a reliable air conditioner control in the different operating situations which is advantageous with respect to the energy consumption, meets the output demands and is simple with respect to the control, specifically also of a CO2 vehicle air conditioner with the load fluctuations typical of the use in a vehicle.
In a further developed expansion element according to the present invention, the control valve of the valve unit influencing the passage cross-section of a fixed throttle is arranged with its low-pressure side fluidically either upstream of an evaporator or between the evaporator and a compressor of the air conditioner, in front of or behind a possibly existing internal heat exchanger.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.